A hybrid three-axis laser Doppler velocimeter system has been demonstrated in our laboratory. The system can monitor the motion of microorganisms in an unconstrained environment. During measurement, a computer system collects and processes time series data from the transit of a microorganism through the measurement volume. The fast Fourier transform of these data contains the motion signature of this microorganism. Because individual microorganisms can be selected
from the field, ambiguity caused by multiscattering among two or more microorganisms can be avoided. With this new system, we can obtain a feature vector that relates to features of the microorganism, such as size, average translational velocity, rotation or wobbling, and flagellum beat frequency. Such a vector appears to be a useful criterion for distinguishing the species using statistical pattern recognition. Successful experiments demonstrate that the new system and technique has some unique advantages. It is possible to search for and characterize specific microorganisms, even at low concentration, in the presence of detritus. Realtime identification of many significant microorganisms should result from further study and improvement.